In certain vehicle brake systems, such as shown and disclosed in U.S. patent application Ser. No. 789,692, filed Oct. 21, 1985, it is necessary to employ a relay valve apparatus which has a first characteristic line for the pressure released into a load circuit while the signal pressure is in effect, and which has a second characteristic line inclined towards lower pressure values for the remaining time. For a relay valve apparatus having a relay piston with two control surfaces, the principal application proposes to render the second control surface ineffective while the signal pressure is not effected by activating a shut-off apparatus which is installed in a pressure connection between the control surfaces.
The principal application has the disadvantage that when the pressure released into the load circuit and follows the second characteristic line, the response resistance which is caused by friction and/or the forces of reset springs between parts moving in relation to each other can only be overcome with increased control pressure.